Oil and gas wells are conventionally drilled by drilling with drill pipe to a certain depth, then running and cementing casing in the well. The operator may then drill the well to a greater depth with drill pipe and cementing another string of casing. In this type of system, each string of casing extends to the surface wellhead assembly, whether on land or subsea.
In some wells, an operator may install a liner rather than an inner string of casing. The liner is made up of joints of pipe in the same manner as casing. Also, the liner is cemented into the well, normally. However, a liner does not extend back to the wellhead assembly at the surface. Instead, it is secured to the last string of casing near the lower end of the casing by a liner hanger.
In most cases, the operator drills the well to the desired depth, retrieves the drill string, then assembles and lowers the liner into the well. A liner top packer may also be incorporated with the liner hanger. A cement shoe with a check valve will normally be secured to the lower end of the liner as the liner is made up. When the desired length of liner is reached, the operator attaches a liner hanger to the upper end of the liner, and attaches a running tool to the liner hanger. The operator then runs the liner into the wellbore on a string of drill pipe attached to the running tool. The operator sets the liner, then pumps cement through the drill pipe, down the liner and back up an annulus surrounding the liner. The cement shoe prevents backflow of cement back into the liner. The running tool dispenses a wiper plug following the cement to wipe cement from the interior of the drill pipe and the liner at the conclusion of the cement pumping. The operator then sets the liner top packer, if used, releases the running tool from the liner, and retrieves the drill pipe.
A variety of designs exist for liner hangers. Some may be set in response to mechanical movement of the drill pipe, including rotation. Others may be set by dropping a ball or dart into the drill string, then applying fluid pressure to the interior of the string after the ball or dart lands on a seat in the running tool. The running tool may be attached to the liner hanger or body of the running tool by threads or by a hydraulically actuated arrangement.
In another method of installing the liner, the operator runs the liner while drilling the wellbore simultaneously. This method has been done in the past but is not commonly employed. A related technology, known as casing drilling, is performed regularly. In casing drilling, a bottom hole assembly at the lower end of the casing includes a drill bit that performs the drilling while the casing is rotated. The operator rotates the casing via a casing gripper at the surface that is suspended from a top drive assembly of a top drive drilling rig. The bottom hole assembly may be retrievable and it may include measuring-while-drilling instruments, directional drilling steering equipment, and a reamer. The bottom hole assembly can be retrieved and rerun by wireline, drill pipe, or pumping the bottom hole assembly down and back up.
Liner drilling differs from casing drilling in that a string of smaller diameter drill pipe is attached to the liner since the liner does not intended extend all the way back to the wellhead assembly at the surface or the subsea wellhead or housing at the sea floor. If the operator wishes to retrieve the bottom hole assembly before cementing the liner, there are no established methods and equipment for doing so. Also, if the operator wishes to rerun the bottom hole assembly and continue drilling with the liner, there are no established methods and equipment for doing so.